implement more caching & public cache api

[WebFreak001]

Allows to have runtime generated bodies with proper cache

For example you can cache runtime diet files with this: (with my runtime diet PR on diet-ng)

import diet.html;

if (handleCacheFile(req, res, "templates/client.dt", "public"))
	return;

res.writeBody(renderStaticHTMLDietFile("templates/client.dt"), "text/html; charset=UTF-8");

[wilzbach]

I am currently very low on time, so I didn't had a chance to look into the RFCs yet, but I already left some first comments. Great work!

[WebFreak001]

(accidentally pushed irrelevant changes I made to fix tests on my machines, broke on linux and windows)

[WebFreak001]

ping @wilzbach @s-ludwig

Tested in firefox and chromium with properly working cache headers, adjusted code to reviews and rebased with master. Can I get another review or merge pls?

[wilzbach]

Looked a bit closer at https://tools.ietf.org/html/rfc7232#section-6 and it doesn't look like you follow it for all parts.

[wilzbach]

Thanks!